feat(welcome): add welcome page for new users after first login

Shows six feature cards and two next steps (Steam, import). The dashboard
redirects to it while Auth settings.welcomeSeenAt is null; "Let's go" and
"Skip" set the stamp. Settings links to it to reopen later.

src/pages/SettingsPage.test.jsx

@@ -88,6 +88,12 @@ describe('SettingsPage public profile', () => {
         await act(async () => auth.answer());
     });
 
+    it('links to the welcome page', async () => {
+        renderPage(profile());
+
+        expect((await screen.findByRole('link', {name: 'SETTINGS.OPEN_WELCOME'})).getAttribute('href')).toBe('/welcome');
+    });
+
     it('turns the public profile off and shows the link while it is on', async () => {
         renderPage(profile());